There is significant interest in multi-spectral imaging (MSI) systems in which 3 or more bands of wavelengths can be captured by an imager for a variety of applications, including remote sensing, health imaging, and industrial sensing. MSI systems operate by capturing light from an object in a number of relatively narrow, discrete spectral bands and directing the light from each spectral band to an imaging sensor. MSI systems can be distinguished from HSI (Hyper-Spectral Imaging) systems that separate light from an object using dispersion to provide continuous spectral content, as contrasted with the discrete spectral bands used by the MSI system. An MSI system can employ any of a number of types of imaging sensors, including both area and linear sensors. Multi-spectral imaging systems can provide high-resolution spectral images of large, extended objects or areas with excellent image quality, short detector acquisition times, and manageable image data sets.
For MSI and related systems, it is necessary to provide some type of filtering mechanism for selecting spectral bandwidths of interest and blocking or attenuating unwanted spectral content. In one type of embodiment, an MSI system may use standard types of color filters, such as conventional dichroic filters, to perform this spectral selection function. Alternatively, various types of tunable transmission filters have been disclosed using liquid crystal (LC) devices, acousto-optical (AO) devices, and tunable Fabry-Perot cavities, for example. Liquid crystal tunable filters have been disclosed in U.S. Pat. No. 5,689,317 entitled “TUNABLE COLOR FILTER” to Miller et al. issued Nov. 18, 1997 and in U.S. Pat. No. 5,892,612 entitled “TUNABLE OPTICAL FILTER WITH WHITE STATE” also to Miller et al., issued Apr. 6, 1999. An imaging apparatus using tunable LC filters is disclosed in U.S. Pat. No. 6,760,475 entitled “COLORIMETRIC IMAGING SYSTEM” to Miller, issued Jul. 6, 2004.
While tunable LC filters provide an effective solution for some imaging applications, these devices have some significant limitations. These limitations include some constraints on spectral range, temperature sensitivity, polarization sensitivity, relatively poor transmission characteristics, and relatively slow response times.
One class of spectral imaging systems employs a spatial light modulator as a type of programmable spectral switch for directing each band of incident light obtained from an object field, in sequence, to a sensor. This approach has been demonstrated successfully for point-imaging and sensing apparatus that utilize a single detector element. However, it can be appreciated that there would be significant advantages to an imaging system in which the programmable spectral switch provides a programmable equivalent to a color filter wheel that would be compatible with both linear and area image sensors. For such a system, it would be particularly advantageous to use a spatial light modulator that is highly efficient, provides high contrast, and operates at high switching speeds. However, the performance requirements for spectral imaging with these devices exceed the capabilities of most types of spatial light modulators.
A particularly advantaged type of spatial light modulator is an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports, as disclosed by Kowarz in U.S. Pat. No. 6,307,663, entitled “Spatial Light Modulator With Conformal Grating Device” issued Oct. 23, 2001. The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of the '663 disclosure has become known as the conformal GEMS device, or more simply as the GEMS device, with GEMS standing for Grating ElectroMechanical System. The GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast, high efficiency, and a relatively large addressable active region. Significantly, the GEMS device is designed for on-axis illumination, unlike other types of high-speed electromechanical light modulators, such as the Digital Micromirror Device™ (DMD) used in Digital Light Processor™ (DLP) systems manufactured by Texas Instruments, Inc., Dallas, Tex. As a further advantage, the GEMS device can be fabricated as a linear device with a thin active area, to modulate a thin line of an image at a time, or can be fabricated with a relatively wide active area in order to modulate a wider segment of an image at one time.
While there would be advantages to the use of GEMS and related devices, their deployment as spectral switches in a programmable spectral imaging system requires a different approach from the conventional use of these components in imaging and display systems.